Roosting behavior may be temperature-driven. Monarchs may not roost, or aggregate as tightly, when temperatures are warm. At the overwintering sites in Mexico, roosting is an anti-predator strategy. Cool temperatures paralyze monarchs, making them vulnerable to predators. A roost provides safety in numbers. Maybe monarchs only shift to roosting behavior when cold temperatures make them vulnerable.
Monarchs were roosting loosely in Sioux Falls, SD on September 3rd. "It's 88 degrees, hot & muggy," said Lynn Hartman.